DESCRIPTION (Verbatim from the applicant's abstract): Histone acetyltransferases are well conserved proteins many of which play critical roles in regulating transcription. Mutations or misregulation of histone acetylation can lead to malignancies, developmental defects and viral pathogenesis. It is however poorly understood as to how transcription is enhanced within a hyperacetylated chromatin region. There have been several hypotheses put forth to explain the mechanisms by which genes are activated by histone acetyltransferases. To better understand these mechanisms, the investigator will use the budding yeast Saccharomyces cerevisiae Gcn5p protein as a model to test the following hypotheses: 1) One or more of the early steps of transcriptional activation is facffitated by the Gcn5p enzymatic action. 2) Gcn5p-acetylated histone tail domains may recruit specific proteins to the hyperacetylated chromatin region which in turn activates transcription. 3) Non-histone proteins may serve as substrates for Gcn5p action. Acetylation of these proteins by Gcn5p plays positive roles in transcription. 4) The two Gcn5p-conta.in HAT complexes in yeast, SAGA and ADA, may perform different functions in regulating HIS3 expression. Specifically, genetic screening for suppressors that rescue the transcription defects due to the lack of Gcn5p HAT activity, for proteins that bind specifically the acetylated histone tails, and for non-histone Gcn5p substrates are proposed. Moreover, individual roles played by the ADA and SAGA complexes in HIS3 expression are to be investigated both genetically and biochemically.